1. Field of the Invention
This invention relates disk drives and to systems and methods for following a track that may be distorted and/or offset from a rotation axis of a disk.
2. Description of Related Art
A conventional hard disk contains one or more disks for data storage and one data head per disk surface for writing and reading data on the surface. In operation, each data head flies above an associated disk surface as the disk rotates and in response to signals from disk drive electronics, reads or writes data in a magnetic medium on the associated surface. FIG. 1 illustrates a typical disk configuration where a data head 120 writes and reads data in concentric circular tracks such as track 110 on disk 100.
Typically, spokelike servo wedges partition each track into data frames such as data frames 106 and 107, where each data frame contains all or part of one or more data sector. Each servo wedge contains one servo sector per track, and the servo sectors contain information identifying a track and a data frame associated with the servo sector. The data head reads information from the servo sectors so that the disk electronics can determine the radial and circumferential position of the data head relative to the disk surface.
A servo system uses the information from the servo wedges when positioning the data head. For example, during an operation often referred to as a seek, the servo system moves the data head radially to a track containing data sector to be accessed. Once on the track, the servo system keeps the data head following the track circumferentially until the desired data sector passes under the data head. If a track is perfectly circular and centered on the disk's rotation axis, no radial movement of the data head is required to keep the data head following the track. However, tracks can be off-center or not circular.
In typical disk drives, clamps are used to mount the disks on a spindle for rotation. Such clamps apply pressure to the disks to hold the disks in place. The clamping pressure may be high enough to prevent starting torque or operational shock and vibrations from displacing the disk relative to the spindle, but high clamping pressure can distort the disks. Accordingly, some disk slippage is acceptable to reduce distortion if the slippage is less than a maximum offset that the servo system tolerates for track following.
FIG. 1 illustrates a track, initially in position 110 and centered on a rotation axis 101, that moves a displacement 130 to a position 111 centered at a point 102 when disk 100 slips. Displacement 130 is referred to herein as the runout. To follow the track as disk 100 spins, the servo system must move data head 120 in and out radially to compensate for the runout when the track is in position 111. U.S. Pat. No. 5,539,714, entitled "Adaptive Runout Compensation for Miniature Disk Drives" describes a disk system that compensates for runout such as illustrated in FIG. 1 and is incorporated by reference herein in its entirety. In U.S. Pat. No. 5,539,714, a disk drive's servo system includes a servo compensator and a secondary servo compensator that generate position signals for positioning a data head over a track. The servo compensator generates a position signal from a position error signal determined for last servo wedge. The secondary servo compensator analyzes the position error signal from previous revolutions of the disk and generates a sinusoidal position signal having a frequency matching the rotation frequency of the disk. The sinusoidal position signal partially compensates for the repetitive variations caused by runout in radial position of the track and has an angular frequency equal to that of the disk's rotation and a phase and magnitude determined from the analysis of the position error signal.
Difficulties in track following can arise for the above-described servo systems if a track is distorted so that the repetitive variation in track radius are not well approximated by a sinusoidal signal having the rotation frequency. In such cases, the secondary servo compensator described above provides an incomplete compensation for repetitive variations, and the servo compensator must provide a larger component of the actuator positioning signal. The servo compensator may not have a sufficient dynamic range to handle the track distortion. An improved servo system is sought that better compensates for periodic variations in track radius caused by both runout and track distortion.